How does SAM calculate the irradiation on a tilted and oriented plan for the PV systems?

I'm so sorry, I mean equation 6.20, the one that is about the reflected irradiance. In the text you say that ground-reflected irradiance is a function of sky diffuse irradiance, but in the equation you use the incident sky diffuse irradiance, am I right?
Thank you!
Raquel

Dear Raquel,
Thank you for the clarification. The text is correct, and the equation is incorrect -- it should be E_d instead of I_d.
Here's what it should be:

The manual is still going through the publication process, so I should be able to get these corrections in before final publication.
Best regards,

Paul.

Hi Paul,

I would like to understand why do we have to inform direct and diffuse radiation separately in PV Watts. Does SAM sum these two inputs to calculate global radiation? I tried to find the equations mentioned by Raquel, but I couldn't find them in the manual.

Many thanks!

Dear Taina,
The equations in the discussion above are from the Photovoltaic Reference Manual, which you can download from the Performance Model Documentation page.
Solar radiation data is typically measured as direct normal and diffuse horizontal irradiance, which is why PVWatts and the detailed photovoltaic model uses those components. The detailed Photovoltaic model can perform calculations to estimate the values of one of those components when it is not available in the weather file, but the PVWatts model does not perform those calculations.
Best regards,

Paul.